Make JSON schema available for verification under https:// URIs

schema/loader.go

@@ -0,0 +1,126 @@
+// Copyright 2018 The Linux Foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package schema
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"strings"
+
+	"github.com/xeipuuv/gojsonreference"
+	"github.com/xeipuuv/gojsonschema"
+)
+
+// fsLoaderFactory implements gojsonschema.JSONLoaderFactory by reading files under the specified namespaces from the root of fs.
+type fsLoaderFactory struct {
+	namespaces []string
+	fs         http.FileSystem
+}
+
+// newFSLoaderFactory returns a fsLoaderFactory reading files under the specified namespaces from the root of fs.
+func newFSLoaderFactory(namespaces []string, fs http.FileSystem) *fsLoaderFactory {
+	return &fsLoaderFactory{
+		namespaces: namespaces,
+		fs:         fs,
+	}
+}
+
+func (factory *fsLoaderFactory) New(source string) gojsonschema.JSONLoader {
+	return &fsLoader{
+		factory: factory,
+		source:  source,
+	}
+}
+
+// refContents returns the contents of ref, if available in fsLoaderFactory.
+func (factory *fsLoaderFactory) refContents(ref gojsonreference.JsonReference) ([]byte, error) {
+	refStr := ref.String()
+	path := ""
+	for _, ns := range factory.namespaces {
+		if strings.HasPrefix(refStr, ns) {
+			path = "/" + strings.TrimPrefix(refStr, ns)
+			break
+		}
+	}
+	if path == "" {
+		return nil, fmt.Errorf("Schema reference %#v unexpectedly not available in fsLoaderFactory with namespaces %#v", path, factory.namespaces)
+	}
+
+	f, err := factory.fs.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	return ioutil.ReadAll(f)
+}
+
+// fsLoader implements gojsonschema.JSONLoader by reading the document named by source from a fsLoaderFactory.
+type fsLoader struct {
+	factory *fsLoaderFactory
+	source  string
+}
+
+// JsonSource implements gojsonschema.JSONLoader.JsonSource. The "Json" capitalization needs to be maintained to conform to the interface.
+func (l *fsLoader) JsonSource() interface{} { // nolint: golint
+	return l.source
+}
+
+func (l *fsLoader) LoadJSON() (interface{}, error) {
+	// Based on gojsonschema.jsonReferenceLoader.LoadJSON.
+	reference, err := gojsonreference.NewJsonReference(l.source)
+	if err != nil {
+		return nil, err
+	}
+
+	refToURL := reference
+	refToURL.GetUrl().Fragment = ""
+
+	body, err := l.factory.refContents(refToURL)
+	if err != nil {
+		return nil, err
+	}
+
+	return decodeJSONUsingNumber(bytes.NewReader(body))
+}
+
+// decodeJSONUsingNumber returns JSON parsed from an io.Reader
+func decodeJSONUsingNumber(r io.Reader) (interface{}, error) {
+	// Copied from gojsonschema.
+	var document interface{}
+
+	decoder := json.NewDecoder(r)
+	decoder.UseNumber()
+
+	err := decoder.Decode(&document)
+	if err != nil {
+		return nil, err
+	}
+
+	return document, nil
+}
+
+// JsonReference implements gojsonschema.JSONLoader.JsonReference. The "Json" capitalization needs to be maintained to conform to the interface.
+func (l *fsLoader) JsonReference() (gojsonreference.JsonReference, error) { // nolint: golint
+	return gojsonreference.NewJsonReference(l.JsonSource().(string))
+}
+
+func (l *fsLoader) LoaderFactory() gojsonschema.JSONLoaderFactory {
+	return l.factory
+}

schema/schema.go

@@ -35,13 +35,36 @@ var (
 	// having the OCI JSON schema files in root "/".
 	fs = _escFS(false)
 
-	// specs maps OCI schema media types to schema files.
+	// schemaNamespaces is a set of URI prefixes which are treated as containing the schema files of fs.
+	// This is necessary because *.json schema files in this directory use "id" and "$ref" attributes which evaluate to such URIs, e.g.
+	// ./image-manifest-schema.json URI contains
+	//   "id": "https://opencontainers.org/schema/image/manifest",
+	// and
+	//   "$ref": "content-descriptor.json"
+	// which evaluates as a link to https://opencontainers.org/schema/image/content-descriptor.json .
+	//
+	// To support such links without accessing the network (and trying to load content which is not hosted at these URIs),
+	// fsLoaderFactory accepts any URI starting with one of the schemaNamespaces below,
+	// and uses _escFS to load them from the root of its in-memory filesystem tree.
+	//
+	// (Note that this must contain subdirectories before its parent directories for fsLoaderFactory.refContents to work.)
+	schemaNamespaces = []string{
+		"https://opencontainers.org/schema/image/descriptor/",
+		"https://opencontainers.org/schema/image/index/",
+		"https://opencontainers.org/schema/image/manifest/",
+		"https://opencontainers.org/schema/image/",
+		"https://opencontainers.org/schema/",
+	}
+
+	// specs maps OCI schema media types to schema URIs.
+	// These URIs are expected to be used only by fsLoaderFactory (which trims schemaNamespaces defined above)
+	// and should never cause a network access.
 	specs = map[Validator]string{
-		ValidatorMediaTypeDescriptor:   "content-descriptor.json",
-		ValidatorMediaTypeLayoutHeader: "image-layout-schema.json",
-		ValidatorMediaTypeManifest:     "image-manifest-schema.json",
-		ValidatorMediaTypeImageIndex:   "image-index-schema.json",
-		ValidatorMediaTypeImageConfig:  "config-schema.json",
+		ValidatorMediaTypeDescriptor:   "https://opencontainers.org/schema/content-descriptor.json",
+		ValidatorMediaTypeLayoutHeader: "https://opencontainers.org/schema/image/image-layout-schema.json",
+		ValidatorMediaTypeManifest:     "https://opencontainers.org/schema/image/image-manifest-schema.json",
+		ValidatorMediaTypeImageIndex:   "https://opencontainers.org/schema/image/image-index-schema.json",
+		ValidatorMediaTypeImageConfig:  "https://opencontainers.org/schema/image/config-schema.json",
 	}
 )
 

schema/validator.go

@@ -67,7 +67,7 @@ func (v Validator) Validate(src io.Reader) error {
 		}
 	}
 
-	sl := gojsonschema.NewReferenceLoaderFileSystem("file:///"+specs[v], fs)
+	sl := newFSLoaderFactory(schemaNamespaces, fs).New(specs[v])
 	ml := gojsonschema.NewStringLoader(string(buf))
 
 	result, err := gojsonschema.Validate(sl, ml)